1. Field of the Invention
The invention relates generally to high-pressure mercury arc lamps, and specifically to lamps with arc loadings in excess of 150 watts/mm, wall loading of more than 0.8 watts/mm, mercury loading pressures of at least 0.16 mg/mm3, and lamp operating-power levels greater than 150 watts. And more in particular where the mercury is less than 0.2 mg/mm3 to reduce pressure to under 200 bar (197 atm), and potassium doping of the tungsten electrodes is used to stabilize the electric arc.
2. Description of the Prior Art
Video projection and fiber illumination systems have benefitted from the ever more powerful arc lamps that have become commercially available over the last several years. Long life is also a critical benefit necessary for the commercial markets. Very often long life and high power levels are at odds with one another.
High operating pressures of 200 bar (197 atm) within mercury lamps operate to concentrate the electrical arc and therefore increase the brilliance of the light output. The addition of sufficient mercury to achieve the highest operating pressure results in a better visual color spectrum output, especially in the reds. Adding halogen helps control or delay envelope blackening, but too much can cause electrode etching.
Akihiko Sugitani, et al., describe a super high pressure mercury lamp in U.S. Pat. No. 6,060,830, issued May 9, 2000, e.g., exceeding 200 bar (197 atm). The lamp has at least 0.16 mg/mm3 mercury and a rare gas. The discharge tube has a tube wall load of at least 0.8 watts/mm2 and includes at least one metal halide with an ionozation potential at most 0.87 times as high as the mercury and added in the range of 2 xc3x9710xe2x88x924 to 7xc3x9710xe2x88x922 xcexcmole/mm3. Such Patent recognizes the arc stability problems and tries to offer a solution.
In general, prior-art ultra high pressure lamps are limited to arc loadings of about 150 watts/mm and maximum power levels of 150-watts. Extreme mercury pressures exceeding 0.2 mg/mm3 of these lamps and high arc temperatures, universally destabilize the arc discharge.
Briefly, a high pressure mercury lamp comprises a quartz envelope that contains an atmosphere and a pair of arc-discharge electrodes. These are coil-wound tungsten that has been doped to grain-stabilize the tungsten crystalline structure, e.g., with potassium or potassium and alumina. Preferred potassium doping levels of the tungsten material are in the range of 5-65 ppm. A suitable commercial product of alumina and potassium doped tungsten material is NON-SAG. The atmosphere generally comprises a rare gas like xenon, to which is added no more than 0.2 mg/mm3 of mercury so as to keep operating pressure under 200 bar (197 atm). But the electrical power applied is sufficient to maintain arc power loadings of at least 150 watts/mm. The resultant wall loading is more than 0.8 watts/mm2, and lamp operating-power levels can be greater than 150 watts.
An advantage of the present invention is that a mercury lamp is provided that operates at slightly lower mercury pressure but maintains luminous efficacy and spectral red content at the higher power level.
Another advantage of the present invention is that a mercury lamp is provided that can operate in excess of 150 watts.
A further advantage of the present invention is that a mercury lamp is provided with higher halogen densities.
A still further advantage of the present invention is that a mercury lamp is provided that has a operational long life.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the drawing figures.